The present invention relates to a multiple access system used in a star type satellite communication system comprising a satellite, one center station (hereafter referred to as a hub station) having a hub function with respect to a large number of small remote earth stations (hereafter referred to as terminal stations) which are respectively connected to individual user terminals via terrestrial cable and which communicate with the hub station via the satellite. In particular, the present invention relates to the improvement of a random access system (RA)/TDMA system) which is one the of time division multiple access (TDMA) systems which can extend from terminal stations to a hub station.
Slotted/Unslotted TDMA using a satellite, reservation, and data retransmission in communication between a hub station and a terminal station are described in the following literature:
(1) A. Fujii et. al., "AA/TDMA-ADAPTIVE SATELLITE ACCESS METHOD FOR MINI-EARTH STATION NETWORKS", Conf. Record, GLOBECOM '86, pp. 1494-1499, Houston, TX. Dec. 1986;
(2) D. Raychaudhuri et al., "Channel Access Protocols for Ku-band VSAT Networks: A Comparative Evaluation", IEEE Communications Magazine, Vol. 26, No. 5, pp. 34-44, May 1988;
(3) N. Abramson, "Packet switching with satellites", National Computer Conference, 1973, pp. 695-702.
(4) Raychaudhuri, "Random Access Communication System with contention Scheduling of Subpacketized Data Transmission and Scheduled Retransmission of Unsuccessful Subpacket", U.S. Pat. No. 4,745,599, issued on May 17, 1988.
Typical forms of the random access scheme are the following two schemes: a pure random access scheme and a slotted random access scheme.
When accepting a datum for transmission from a terminal connected with a station in the pure random access scheme, respective stations transmit data formed into data packets on satellite channels without any attempt to coordinate such transmission with transmissions of other stations. In this scheme, data packets, which are transmitted from respective stations and which are not synchronized with each other, often partially overlap and interfere with each other on the satellite, resulting in destroyed data. On the other hand, a slotted random access scheme has a lower probability of data destruction compared to the above-mentioned pure, namely, unslotted random access scheme. In this scheme, each terminal station transmits data packets in synchronization with a time reference as transmitted from the hub station in order to avoid partial overlap. Supposing in consideration of a difference between the two schemes that the data length is constant and data are generated in accordance with the Poisson's process, the throughput, which is the transmission
efficiency or the throughput efficiency, is approximately 37%, or approximately twice in the slotted random access scheme as compared with approximately 18% in the unslotted random access scheme. The throughput is expressed in an average percent of channel capacity carrying actual user data. In the slotted random access scheme, the terms "collision" and "collide" are often used instead of the terms "interference" and "interfere".
In the above described slotted random access scheme, however, further improvement of the throughput is desired. When data destruction due to collision of data packet occurs stochastically, the data must be retransmitted and hence the average delivery time of data becomes longer. Especially in the case of a transmission method in which a datum exceeding the time slot length is divided into a plurality of time slot length data and a second divided datum is transmitted after ascertaining reception of the answer back of the signal acknowledging of a first transmitting divided datum the delivery time of all the divided datum becomes longer. For a system requiring running with a high throughput and a system in which long data exceeding the time slot length must be accurately delivered in a short time, a reservation scheme obtained by incorporating a time slot reservation function into the conventional slotted random access scheme is considered.
The reservation scheme is basically a scheme in which time slot reservation is performed by some method before data transmission. The reservation schemes are roughly classified in accordance by way of reservation into two kinds of reservation schemes, i.e., an explicit reservation scheme and an implicit reservation scheme. In the explicit reservation scheme, time slot reservation is performed by using a reservation packet. In the implicit reservation scheme, a reservation packet is not used and once a data packet has been delivered successfully without collision, identical time slots in succeeding frames are regarded as automatically reserved. The term "reservation scheme" typically refers to the explicit reservation scheme. The present invention also refers to the explicit reservation scheme.
When data are emergently transmitted from a terminal station to the hub station by using the conventional slotted random access scheme and data cannot be successfully received by the hub station due to collision of the emergent data packet, the data are retransmitted until the emergency data can be successfully received by the hub station.
When a reservation scheme based upon the above described conventional slotted random access scheme is used and the necessity of reservation occurs at a terminal station, one packet used exclusively for a reservation request or one packet with reservation request information added must first be transmitted. In case the reservation packet cannot be successfully received by the hub station due to collision, the reservation packet must be retransmitted from the terminal station many times until the reservation packet can be successfully received by the hub station. This results in a problem that additional time is required for that purpose.
The similar necessity holds true for emergency data transmission as well. Until an emergency data packet can be successfully received by the hub station, the emergency data packet must be retransmitted from a terminal station many times, resulting in the problem of additional time requirements.